Teachers’ unions and school performance: Evidence from California charter schools

•Estimates the impact of teacher unionization on student achievement in California charter schools between 2003 and 2013.•We compare changes in student achievement in schools that unionize to those that do not using difference-in-differences and event study models.•We find a positive impact of unionization on math achievement of about .17 standard deviations.•Point estimates on English achievement are positive but smaller at .06 standard deviations, and not statistically significant.